The broadest objective is to understand the visual system and how it works, so that pathologies can be better detected, understood, prevented, and treated. Psychophysical techniques are used because of their non-invasiveness; and the observations are based mainly on thresholds and similar measures, so as to minimize the assumptions necessary to draw conclusions about the processing and transmission of information by the visual system. The experiments are intended to describe the operations performed by the visual system on the information it receives, and to constrain admissable representations of the sequences of such operations. The operations studied include transformations of stimulus intensity and filtering of spatial information.